We designed a computer-aided design and processing system for sheet metal products for a sheet flexible manufacturing system of a technology company in Shenyang. The system consists of three subsystems: CAD, data interface, and CAPP. The sheet metal CAD subsystem is based on the general drawing software AutoCAD for secondary development; the CAD/CAPP data interface subsystem analyzes and processes the graphics information of the CAD subsystem and transfers it to the CAPP subsystem to complete the internal data connection of CAD/CAPP. Online delivery of information and real-time processing.
Figure 1 System composition diagram
1 Sheet Metal Product Graphical Data Features
The sheet metal product CAD subsystem is designed based on the parametric idea. As long as several key parameters are entered, the system automatically generates a three-dimensional view and an expanded view of the sheet metal part. The expanded image is a two-dimensional graphic. The CAD/CAPP data interface processes the *.DXF command group file of the expanded image and passes the information to the CAPP subsystem to meet the blanking of the board.
Figure 2 sheet metal design
The design and processing of sheet metal products have their own structure, materials and process characteristics.
1) Expanding Drawings Two-dimensional graphics consist of lines, arcs, and circles. All geometric features are on a plane. (The quadratic and cubic curves consist of arcs or linear interpolations).
2) All process features of blanking, such as roughness, positioning accuracy, straightness, etc. are guaranteed by the NC machine.
The CAD system information model is a geometric model, wire rack model, face frame model and voxel modeling. The tolerance and technical requirements are handled according to the label information. The CAPP system requires process features and manufacturing information, not geometric features of points, lines, and surfaces. And due to its own characteristics of sheet metal product design and processing, there are many problems with the *.DXF file as a data file directly as input information for the CAPP subsystem.
The existence of a large number of non-processing information sheet metal expansion system is developed under the AutoCAD software environment. The *.DXF command group file transmitted from the CAD subsystem contains all the information of the standard parts diagram, including dimension lines, auxiliary lines, etc. Processing information; making the operation of CAPP subsystem, it is easy to produce the processing ring is not clear, generate the wrong NC operation instructions.
• Geometric intersections do not coincide. Since the geometric point information in the *.DXF file is obtained by CAD graphics through internal calculations, the geometric intersection of many graphic elements is actually two points. Although the error between the two points is small and does not affect the machining accuracy, such graphical data cannot be used for automatic process programming in the CAPP subsystem.
. Topological disorder due to * .DXF files, CAD graphics-related data in order to generate graphics subsystem elements in AutoCAD graphical interface and storage and therefore can not guarantee the topological relationships between graphical elements whether graphical parts With precise logic. If closed loops cannot be guaranteed, the topological relationships between the graphical elements that make up the loop cannot be guaranteed. This graphic cannot be used for CNC machining.
Lack of programmed rotation direction determining process to determine the processing to be punch punches for machining the inner contour or the outer contour. However, the contour information contained in the *.DXF file of the unfolded figure cannot determine the orientation of the ring. The group ring entity must be analyzed and calculated, and the entities in the ring are sequentially written into the data file according to the processing direction required by the output of the data file.
2 Solution
2.1 Eliminating Problems within the CAD Subsystem <br> Consider CAPP requirements when designing the overall CAD subsystem.
2.2 Establishing an Independent Data Interface <br> The idea of ​​this method is to make full use of the AutoCAD development platform in the CAD subsystem, temporarily not considering the requirements of the CAPP subsystem. For the CAPP subsystem, make a data interface software whose task is to further process the *.DXF file of the part development drawing, including: Excluding processing information (including unnecessary process information and technical specifications); Coincidence points, establish topological relations, find processing rings; according to prior agreement, point out the direction of processing loops.
3 design ideas and algorithm implementation
In the sheet metal product CAD subsystem, the part development diagram first uses the AutoCAD EXPLODE command to break the graphics and generate the corresponding *.DXF file. Only from the *.DXF extract the solid segment information (CIRCLE, LINE, ARC) related to the workpiece profile data; perform analysis and calculation to find a point on the outer contour of the graphic information, according to this point to find the outer contour processing ring; Find the remaining inner contour processing ring (CIRCLE as an independent processing ring) in the remaining graphics information; judge the loop direction, and write the graphics information contained in the processing loop into the data output file according to the loop.
Figure 3 flow diagram
3.1 The data structure of the system
The system data structure uses a doubly linked list structure. Because of the sheet metal processing characteristics (sheet uniformity), it can be easily handled in two-dimensional graphics. Moreover, whether it is punching or blanking, its contours form a closed loop, which is a figure (or a full circle) formed by connecting lines or arcs end to end.
set up
Class ARC:public CIRCLE,public LINE
{
Int Direct;
};
Among them, the class ARC is a subclass of CIRCLE, LINE.
Members of class CIRCLE, LINE: Center represents the coordinates of the center of the circle; Radius represents the radius; Start, End are the coordinates of the starting point and ending point of a straight line or an arc; Direct is a unique member of the ARC and indicates the direction of the arc: Clockwise is equal to +1 and arc is counterclockwise equal to -1.
3.2 The geometric information extraction module <br> The graphic entity has been broken into three types: "LINE", "ARC", and "CIRCLE". After research, it is found that the V13.0 graphics entity is broken to the last line, ARC, CIRCLE, such as the fitting of the Pline by Fit, to Arc, and the Pline-fit Pline to the Line, etc.). Therefore, the core of the program is ARC, LINE, and CIRCLE. The program calls an entity from the ENTITIES of the *.DXF file to determine whether it is LINE or ARC or CIRCLE, and then extracts the geometric information according to its group code after streaming.
For example: For a straight line, you can use 10 and 11 group codes to extract the starting and ending coordinates.
3.3 Group Ring Algorithms There is a Distant() function in each class. This function is used to find the maximum distance from the origin of each entity. Compare the Distant() function value of all entities and find the maximum distance from the origin. The entity N1 corresponding to this point must be on the outer contour.
If N1 is a circle, this part outside the processing ring S0 is this circle.
If N1 is a straight line or an arc, the group S0 (outer contour machining ring) is referenced to the entity N1. The starting point of N1 is the starting point of ring S0, and the starting point of N1 is the next entity of ring.
Search for the start and end points of the remaining entities (consider only straight lines and arcs) to find approximate coincident points (when the difference between the point and the point is smaller than a small integer ε, consider it a coincidence point, and mesh the geometric intersection) as the adjacent two The intersection of basic graphic entities. The entity where this point is located acts as the second chain of the ring and continues to look for the next chain of rings in the remaining entities ARC and LINE, and to determine whether the termination point in the chain is approximately coincident with the start of the ring. Do not repeat this process, otherwise the ring of the outer contour ring S0 ends.
In the remaining ungrouped entities ARC, LINE, the group rings S1, S2, S3, S4,... are continued (all inner contour rings and the group ring method is the same as above). Until all entities ARC, LINE are grouped into each ring.
Read the remaining entity CIRCLE as an inner ring.
3.4 Data Output Modules <br> Determine the direction of the loop at the time of output. For a ring consisting of three entities, the direct two entities multiply and use the right-hand rule to judge the direction of the ring; for the general entity ring to find the point in the ring, the straight line carries out the cross-fork calculation to judge the direction of the ring, and then outputs the data to the data according to the agreed format. In the file, transfer the data file to the CAPP subsystem.
CAD/CAPP integration is actually a combination of structural design modules and process technology modules. The CAD/CAPP data interface subsystem converts the graphic information output by the CAD module into a data format in which the CAPP can receive the recognition, and realizes the extraction, exchange, sharing, and processing of information between the CAD and the CAPP. The establishment of the CAD/CAPP data interface subsystem of sheet metal products enables the system to make full use of the secondary development functions of the AutoCAD platform, realizes the parameterization, standardization, serialization, automation of sheet metal product design and manufacturing, and shortens the production cycle of new products. Increase productivity.
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